1. Dust and scratches. Hopefully there aren't scratches, but all the blowing in the world seems to leave a few particles behind that you need to take care of.
2. Color balance. Colors fade at different rates so the older the slide the more you need to handle color cast. I find my slides tend towards blue.
3. Grain. You can't get away from grain, but I find it is difficult to get good sharpness without enhancing the grain that is present.
4. Technique. There are a lot of variables to consider. You can use all kinds of automated tools with my scanner including curves and other corrections that can also be applied in Photoshop. You must decide to apply them in the scan (which saves time) or later in Photoshop for more control.
5. Poor camera technique. I have grown in leaps and bounds since going digital. One of the bigs things I took some time to figure out early on is the lowest speed I can handhold at. I notice when scanning old images from film that I used to get quite a bit more camera shake 10 years ago...
My scanner is a Minolta DImage IV, which I don't think you can buy anymore. Fortunately it still works on my computer, but I haven't found any updated drivers and haven't been able to get it working on any other computer. It does a great job and is much, much better than a flatbed I tried before it. It uses a 4 slide cartridge or 6 image negative holder. You cannot scan anything else with it...
We'll see how this image does. I suspect I'm better off taking new shots than going through old slides :)